The $2 billion Facebook deal announced last month for Irvine startup Oculus VR still hasn't closed, but the former employer of 3D engineer John Carmack appears to be looking for a cut.

ZeniMax Media claims Carmack took intellectual property with him to the virtual-reality company. He became chief technology officer at Oculus last August but continued at ZeniMax until November.

In a statement, ZeniMax said it sent letters in recent weeks to Oculus, claiming it has rights to "extensive VR research and development works done over a number of years by John Carmack."

"No work I have ever done has been patented," Carmack responded on Twitter. "ZeniMax owns the code that I wrote, but they don't own VR."

A statement provided by Oculus said it planned to "vigorously defend Oculus and its investors."

"When there's this type of transaction, people come out of the woodwork with ridiculous and absurd claims," the statement read.

Carmack is a pioneering developer who helped usher into reality concepts like networked multiplayer and first-person perspective 3D games while at the company he co-founded, Id Software. ZeniMax bought Id in 2009 and along with it game franchises Carmack helped build, including "Doom" and "Wolfenstein."

After decades of false starts, Carmack reignited VR buzz in 2012 when he demonstrated at a gaming conference a duct-taped prototype of the Rift headset built by Long Beach teen Palmer Luckey. The two had connected online at VR enthusiast forums and traded notes.

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"A lot of the things we're doing weren't invented by us," Luckey told the Register late that year. "They were invented by other people. And we happen to have the luck to be in the right decade to make it happen."

The first game intended to run on the $300 Oculus Rift development kits was Id's "Doom 3," which Carmack had integrated into the headset to demonstrate how the Rift can make its wearer feel like they are the main character in a video game.

But their plan never moved beyond the demonstration; Oculus ended up offering a credit or refund in place of the game. Carmack's side venture, an aerospace company, ran out of money in 2013 and his efforts to work on VR while at ZeniMax didn't pan out.

Earlier this year Carmack told USA Today in a rare interview, "When it became clear that I wasn't going to have the opportunity to do any work on VR while at Id Software, I decided not to renew my contract."

ZeniMax claims Luckey "acknowledged in writing" ownership of the alleged intellectual property, and Oculus has used it without authorization or compensation.

The Wall Street Journal reported that ZeniMax had sought compensation as early as August 2012. Oculus was still raising money on the crowdfunding website Kickstarter at that time, while negotiations with ZeniMax continued "on and off for nearly six months," according to the report.

An equity stake in Oculus was once on the table but the parties were "unable to reach a satisfactory resolution," the ZeniMax statement said. The company said it would "take the necessary action to protect its interests."

Facebook's acquisition of Oculus was to close by the end of the second quarter.